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Just to ensure that we are discussing the same thing, the interest that was sold (the 1/4 interest that was held by the stepsons) - was that a "tail estate" - meaning that this was the ownership in what we call "Fee Simple" (or what may be termed "actual ownership") of the property (as opposed to the life estate)?
Because, the way you describe the situation, I would suspect the following:
Father leaves life estate to stepmother. Stepmother can do what she likes with her life estate (including selling her life estate unless there is some prohibition against doing so). However, she can only sell the life estate. So any purchasers of the life estate are only entitled to that limited ownership, once the stepmother dies, the life estate ends (so the buyer's interest would terminate).
Father would have a remainder (split into quarters as you describe). The remainder would give the property "fee simple" to these individuals.
If my suspicion is correct, stepmother can only sell her children's interest if they are minors and she is managing their affairs.
(So if stepmother is managing minor children's property interest in their 1/4 interest of the fee simple, she could sell that on their behalf to another individual, such as one of the other 1/4 interest holders making them a 1/2 interest holder). None of this would be dependent on or affect the life estate.
However, if the stepchildren are adults and did not give consent for this transaction it is "void ab initio" (void as soon as it was made) as she has no authority to sell something that does not belong to her.